Formulations and processing of cementitious components to meet target strength and co2 uptake criteria

ABSTRACT

Provided herein are compositions and methods of carbonation processing for the fabrication of cementitious materials and concrete products. Embodiments include manufacturing processes of a low-carbon concrete product comprising: forming a cementitious slurry including portlandite; shaping the cementitious slurry into a structural component; and exposing the structural component to a CO 2  waste stream, thereby enabling manufacture of the low-carbon concrete product.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/821,478, filed Mar. 17, 2020, which claims the benefit of U.S.Provisional Patent Application No. 62/819,895, filed Mar. 18, 2019, eachof which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumbersDE-FE0029825 and DE-FE0031718, awarded by the Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND

Traditional concrete is a mixture of calcium silicate-dominant ordinaryportland cement (“OPC”), mineral aggregates, water, and chemicaladditives. The reaction of OPC with water (hydration) forms calciumsilicate hydrate (C-S-H) compounds. The precipitation of C-S-H betweenproximate particles induces cohesion/hardening, and the resultingporosity reduction and refinement strengthen the concrete. Due to thesignificant impact of the construction industry on climate change, thereis a pressing demand to implement OPC-alternative cementation solutionswith significantly reduced embodied CO₂ intensities. Over 30 billionmetric tons of concrete are produced per year, involving the productionof over 4.5 billion metric tons of cement, with CO₂ emissions intensityon the order of 0.8-0.9 kg CO₂/kg cement. Emissions associated withcement production make up over 5% of global CO₂ emissions, contributingsignificantly to global climate change.

It is against this background that a need arose to develop theembodiments described herein.

SUMMARY

Provided herein are manufacturing processes for forming cementedsiliceous solids.

Some embodiments of the present disclosure include a manufacturingprocess of a low-carbon concrete product, comprising: forming acementitious slurry including portlandite; shaping the cementitiousslurry into a structural component; and exposing the structuralcomponent to a post-combustion or post-calcination flue gas streamcontaining CO₂, thereby enabling manufacture of the low-carbon concreteproduct. In some embodiments, forming the cementitious slurry includescombining water and a binder including the portlandite (e.g., as theprimary feedstock), and optionally cement and coal combustion residuals(e.g., fly ash) at a water-to-binder mass ratio (w/b) of about 0.5 orless. In some embodiments, w/b is about 0.45 or less, about 0.4 or less,about 0.35 or less, or about 0.3 or less, and down to about 0.25 orless. In some embodiments, forming the cementitious slurry includescombining water and a binder including a cement, portlandite, and coalcombustion residuals, at a mass percentage of the cement in the binderof about 25% or greater and up to about 50%. In some embodiments, themass percentage of the cement in the binder is about 30% or greater,about 35% or greater, about 40% or greater, or about 45% or greater, andup to about 50%. Some embodiments, further comprise drying thestructural component prior to exposing the structural component tocarbon dioxide. In some embodiments, drying the structural componentincludes reducing a degree of pore saturation (S_(w)) to less than 1. Insome embodiments, S_(w) is about 0.9 or less, about 0.8 or less, about0.7 or less, about 0.6 or less, about 0.5 or less, or about 0.4 or less,and down to about 0.1. In some embodiments, drying the structuralcomponent includes reducing S_(w) to a range of about 0.1 to about 0.7,about 0.2 to about 0.6, about 0.2 to about 0.5, or about 0.2 to about0.4. In some embodiments, drying the structural component is performedat a temperature in a range of about 20° C. to about 85° C., about 30°C. to about 65° C., or about 35° C. to about 55° C., for a time durationin a range of 1 h to about 72 h. In some embodiments, a green bodystructural component is produced either by compacting the cementitiousslurry (e.g., dry-casting) or by pouring the slurry in to a mold (e.g.,wet-casting) to form the structural component. In some embodiments,compacting the cementitious slurry includes reducing S_(w) to lessthan 1. In some embodiments, Sw is about 0.9 or less, about 0.8 or less,about 0.7 or less, about 0.6 or less, about 0.5 or less, or about 0.4 orless, and down to about 0.1. In some embodiments, compacting thecementitious slurry includes reducing S_(w) to a range of about 0.1 toabout 0.7, about 0.2 to about 0.6, about 0.2 to about 0.5, or about 0.2to about 0.4. In some embodiments, compacting the cementitious slurry isperformed at a pressure in a range of about 0.5 MPa to about 50 MPa. Insome embodiments, exposing the structural component to carbon dioxide isperformed at a temperature in a range of about 20° C. to about 85° C.,about 30° C. to about 75° C., about 35° C. to about 70° C., or about 40°C. to about 65° C. In some embodiments, the low-carbon concrete producthave up to 75% lower embodied carbon intensity than a traditionalcement-based concrete product. In some embodiments, the lower carbonintensity is due to (a) partial substitution of cement with portlanditeand/or fly ash and/or (b) CO₂ uptake during manufacturing,

Some embodiments of the present disclosure include manufacturing processof a low-carbon concrete product, comprising: providing a targetcompressive strength of the concrete product; providing a predictionmodel relating carbon dioxide uptake to compressive strength; forming acementitious slurry including portlandite; forming the cementitiousslurry into a structural component; and exposing the structuralcomponent to carbon dioxide, thereby forming the low-carbon concreteproduct, wherein exposing the structural component to carbon dioxideincludes monitoring carbon dioxide uptake of the structural component,and exposing the structural component to carbon dioxide is performed atleast until the carbon dioxide uptake of the structural component isindicative of meeting the target compressive strength according to theprediction model. In some embodiments, the carbon dioxide is containedwithin a post-combustion or post-calcination flue gas stream

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows portlandite and fly ash (FA) particles that demonstratesigmoidal trends in final conversion X_(f) that increase with relativehumidity RH and are independent of temperature T and CO₂ concentration[CO₂].

FIG. 2A shows the time-dependent CO₂ uptake of wet-castportlandite-enriched mortar specimens (about 50 mm×about 100 mm; d×h)prepared to different initial levels of liquid water saturation S_(w) inpores. FIG. 2B shows the dependence of compressive strength on CO₂uptake for the same mortars show in FIG. 2A.

FIG. 3A shows the compressive strength evolution of portlandite-enrichedand reference mortars (about 75% portland cement and about 25% Class Ffly ash) during carbonation processing and limewater curing. FIG. 3Bshows the evolution of calcium hydroxide content, and FIG. 3C showsnon-evaporable water content of the same specimens. Processing involvedeither: drying at about 45° C. for about 12 h prior to about 12-hcarbonation at about 45° C., or drying at about 45° C. for about 24 hbefore limewater curing.

FIG. 4A shows CO₂ uptake after 60 h CO₂ exposure C(60 h) as a functionof initial saturation S_(w) for wet-cast composites, dry-castcomposites, and portlandite compacts. In all cases, reducing S_(w)enhanced CO₂ uptake for S_(w)>0.10. Carbonation occurred in 12% CO₂[v/v] at 22° C. FIG. 4B shows dependence of the 24-h carbonation ofdry-cast portlandite pellets on S_(w) illustrates a similar criticalS_(w), regardless of the pellets' relative density (ρ/ρ_(s)).

FIG. 5A shows compressive strength of non-carbonated and carbonated (forabout 6 or about 60 h) dry-cast mortar specimens as a function of theirrelative density. FIG. 5B shows compressive strength (left y-axis) andCO₂ uptake (right y-axis) of dry-cast portlandite-enriched mortarsfollowing exposure to CO₂ (about 12% CO₂) at varying reactiontemperatures for a range of about 1 h to about 72 h, e.g., about 24 h.Of the temperatures tested, about 65° C. is an optimal temperature formaximizing both CO₂ uptake and compressive strength. At highertemperatures, drying during carbonation reduces the liquid watersaturation (S_(w)) below the critical value for carbonation ofportlandite.

FIG. 6 shows the extent of carbonation strengthening (the ratio betweenthe strength of carbonated and non-carbonated specimens) for wet-castand dry-cast mortars as a function of their CO₂ uptake.

FIG. 7 shows compressive strength evolution as a function of time fortwo wet-cast mortars, one which was dried at about 45° C. for about 12 hprior to about 24-h carbonation (red circles) and another which wasdirectly carbonated for about 36 h (blue triangles). The wet-castmortars were composed of: about 35 mass % portlandite, about 15 mass %Class fly ash, and about 50 mass % OPC at water-to-binder ratio (w/b) ofabout 0.40. Carbonation processing was carried out using about 12% CO₂[v/v].

FIG. 8A shows time-dependent CO₂ uptake within wet-cast composites andFIG. 8B shows time-dependent CO₂ uptake within dry-cast composites atvarying initial saturation S_(w). The data was fit by an equation of theform C(t)=C(t_(u))[1−exp((−kt)/C(t_(u)))] to estimate the apparentcarbonation rate constant k(h⁻¹). FIG. 8C shows the CO₂ uptake after 60h CO₂ exposure C(60 h) as a function of initial saturation S_(w) forwet-cast composites, dry-cast composites, and portlandite compacts. Inall cases, reducing S_(w) enhanced CO₂ uptake for S_(w)>0.10.Carbonation occurred in 12% CO₂ [v/v] at 22° C.

FIG. 9A shows total moisture diffusivity of dry-cast/wet-cast compositesas a function of their initial saturation S_(w). The dashed line is aguide for the eye. FIG. 9B shows apparent carbonation rate constant as afunction of the total moisture diffusivity across dry-cast and wet-castcomposites. FIG. 9B shows 60-h CO₂ uptake as a function of thenon-evaporable water content w_(n)/m_(OPC) of dry-cast and wet-castcomposites following carbonation. The degree of OPC hydration for thedry-cast and wet-cast composites were estimated between 9.1%-20.8% and45.2%-60.1%, respectively, using thermogravometric analysis (TGA). Thelower and upper bounds of w_(n)/m_(OPC) in FIG. 9C for each data pointcorrespond to values at t=0 h and t=60 h of carbonation, respectively.The specimens with initial S_(w)<S_(w,c) are excluded in FIG. 9C.

FIG. 10A shows the evolution of compressive strength as a function ofCO₂ uptake for wet-cast composites at varying initial saturation levelsinduced by drying and FIG. 10B shows the evolution of compressivestrength as a function of CO₂ uptake for dry-cast composites at varyinginitial saturation levels induced by compaction pressure. The dependenceof the slope of FIG. 10C the strength per unit CO₂ uptake as a functionof the change in non-evaporable water content (Δ(w_(n)/m_(OPC))) duringCO₂ exposure, and FIG. 10D the strength per fraction of OPC hydrationw_(n)/m_(OPC) as a function of the CO₂ uptake during CO₂ exposure fordry-cast and wet-cast composites. Extrapolation of these trends was usedto assess the independent contributions of hydration or carbonation. Inall cases, carbonation was carried out using 12% CO₂ [v/v] at 22° C.

FIG. 11A shows carbonation strengthening factor as a function of theultimate CO₂ uptake for wet-cast and dry-cast composites. FIG. 11B showsthe evolution of 24-h compressive strength and CO₂ uptake as a functionof the reaction temperature for dry-cast composites. Herein, carbonationwas carried out using 12% CO₂ [v/v].

FIG. 12A shows the evolution of compressive strength, FIG. 12B shows theevolution of normalized non-evaporable water content, and FIG. 12C showsthe evolution of normalized calcium hydroxide content in wet-castportlandite-enriched (CH-OPC-FA) and portlandite-free (OPC-FA)composites during drying, carbonation, and limewater curing. The resultsof non-carbonated specimens with and without portlandite are also shownfor comparison. The carbonated specimens were dried at 45° C. for 12 hthen exposed to CO₂ for 12 h at 45° C., whereas the non-carbonatedspecimens were dried at 45° C. for 24 h before limewater curing.Carbonation was carried out using 12% CO₂ [v/v]. As indicated in FIG.12A, the portlandite-enriched (CH-OPC-FA) composites featured 4.3×higher CO₂ uptake than portlandite-free (OPC-FA) composites. The“pink-shaded” and “blue-shaded” regions indicate vapor-phase processing(drying and carbonation) and limewater curing durations.

FIG. 13 shows particle size distributions of solids as determined bystatic light scattering (for binder materials) and sieve analysis (forsand). The median particle diameters (d₅₀) were 3.8 μm, 8.9 μm, and 17.2μm for the portlandite, fly ash, and OPC, respectively.

FIG. 14 shows a schematic of the drying and carbonation apparatusshowing the flow-through reactors and related online instrumentation.The experiments were carried out at ambient pressure (p of about 1 bar).

FIG. 15A shows the time-dependent evolution of water saturation level,S_(w), for wet-cast composites at different drying temperatures and FIG.15B shows the time-dependent evolution of water saturation level, S_(w),for wet-cast composites at different drying air flow rates. FIG. 15Cshows compaction-dependent S_(w) for the dry-cast composites at varyingcompaction levels.

FIG. 16 shows a correlation between compressive strength development andnon-evaporable water content for wet-cast composites across increasingcarbonation duration.

FIG. 17 shows a dependence of the compressive strength on w_(n)/m_(OPC)during drying before CO₂ exposure across diverse drying conditions.

FIG. 18A provides a representative SEM micrographs showing surfacemorphology and formation of carbonate products (identified by SEM-EDSanalysis) for wet-cast composite (w_(n)/m_(OPC)=11.5%) and FIG. 18Bprovides a representative SEM micrographs showing surface morphology andformation of carbonate products (identified by SEM-EDS analysis) fordry-cast composite (w_(n)/m_(OPC)=4.8%).

DETAILED DESCRIPTION

Embodiments of this disclosure are directed to compositions and methodsof carbonation processing for the fabrication of cementitious materialsand concrete products that meet design criteria of compressive strengthand CO₂ uptake. Compressive strength is a design criterion thatindicates the mechanical performance of concrete materials andpre-fabricated concrete products (e.g., concrete masonry units, beams,slabs, and so forth). The CO₂ uptake (quantified as a mass of CO₂incorporated into solid products per mass of initial solid material)describes the material's efficiency in sequestering gaseous CO₂ intostable solids. Enhancing CO₂ uptake reduces a material's embodied CO₂emissions footprint, and allows impactful removal of gaseous CO₂ fromindustrial emissions sources. Together, these metrics describe thefundamental design criteria for producing construction products withcarbonate-based binders that incorporate alkaline solid wastes and fluegas CO₂ streams.

In an aspect according to some embodiments, a manufacturing process of alow-carbon concrete product includes: (1) forming a cementitious slurryincluding portlandite; (2) shaping the cementitious slurry into astructural component; and (3) exposing the structural component to a CO₂waste stream, such as a post-combustion or post-calcination flue gasstream containing carbon dioxide, thereby enabling manufacture of thelow-carbon concrete product. It is understood that, in some embodiments,the amount of carbon dioxide in the CO₂ waste stream (e.g.,post-combustion or post-calcination flue gas stream) is greater thanconcentration of carbon dioxide typically in the atmosphere.

In some embodiments, the process operates, effectively, at ambientpressure and/or gas temperatures. For example, in come embodiments, step(3) is performed at an ambient pressure. In some embodiments, thepressure is about 0.5 to about 10 atm, e.g., about 0.5, 0.6, 0.7, 0.8,0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 3, 4, 5, 6, 7,8, 9 or 10 atm. In some embodiments, step (3) is performed at an ambienttemperature. in some embodiments, the temperature is about 15° C. toabout to about 80° C., e.g., about 15, 20, 25, 30, 35, 40, 45, 50, 55,60, 65, 70, 75 or 80° C.

In some embodiments, forming the cementitious slurry includes combiningwater and a binder including the portlandite, and optionally cement andcoal combustion residuals (e.g. fly ash) at a water-to-binder mass ratio(w/b) of about 0.5 or less, about 0.45 or less, about 0.4 or less, about0.35 or less, or about 0.3 or less, and down to about 0.25 or less.

The term coal combustion residuals has its typical meaning in the art.Coal combustion residuals can include coal ash, and can includecomponents such as those residuals produced when coal is burned by powerplants. Coal ash can include one or more of fly ash, bottom ash, andboiler slag. Fly ash is generally composed mostly of silica and can bemade from the burning finely ground coal.

A post-combustion or post-calcination flue gas stream can be producedfrom coal fired power plants, and can include, e.g., 12.7% CO₂, 2.5% 02,66.7% N₂+Ar, 18.1% H₂O, 23 ppm SO₂, and 28 ppm NO_(x). Furthermore, theportlandite carbonation and CO₂ mineralization reaction is insensitiveto the presence of acid gases (e.g., SO_(x) and NO_(x)) that may becontained in flue gas streams. In some embodiments, the post-combustionor post-calcination flue gas stream can be simulated flue gas, e.g., agas stream that is the same or similar to a post-combustion orpost-calcination flue gas stream from an industrial process, such asfrom coal fired power plants. In some embodiments, the post-combustionor post-calcination flue gas stream includes carbon dioxide in an amountof about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or20, up to 50%. In some embodiments, the CO₂ waste stream, such as thepost-combustion or post-calcination flue gas stream, is diluted. Forexample, the stream may be diluted by 10, 20, 30, 40, 50, 60, 70, 80, or90 percent from its original concentration. In some embodiments, the CO₂waste stream, such as the post-combustion or post-calcination flue gasstream, is enriched. For example, the stream may be enriched by 10, 20,30, 40, 50, 60, 70, 80, or 90 percent from its original concentration

In some embodiments, forming the cementitious slurry includes combiningwater and a binder including a cement, portlandite, and coal combustionresiduals at a mass percentage of the cement in the binder of about 25%or greater, about 30% or greater, about 35% or greater, about 40% orgreater, or about 45% or greater, and up to about 50%.

In some embodiments, the manufacturing process includes drying thestructural component prior to exposing the structural component tocarbon dioxide. In some embodiments, drying the structural componentincludes reducing a fraction of pore volume that is saturated withliquid water (S_(w)) to less than 1, such as about 0.9 or less, about0.8 or less, about 0.7 or less, about 0.6 or less, about 0.5 or less, orabout 0.4 or less, and down to about 0.1. In some embodiments, dryingthe structural component includes reducing S_(w) to a range of about 0.1to about 0.7, about 0.2 to about 0.6, about 0.2 to about 0.5, or about0.2 to about 0.4. In some embodiments, drying the structural componentis performed at a temperature in a range of about 20° C. to about 85°C., about 30° C. to about 65° C., or about 35° C. to about 55° C., for atime duration in a range of 1 h to about 72 h.

In some embodiments, shaping the cementitious slurry includes compactingthe cementitious slurry to form the structural component. For example,in some embodiments, shaping the cementitious slurry includes eithercompacting the cementitious slurry (dry-casting) or pouring the slurryin to a mold (wet-casting) to form the structural component. In someembodiments, compacting the cementitious slurry includes reducing adegree of pore water saturation (S_(w)) to less than 1, such as about0.9 or less, about 0.8 or less, about 0.7 or less, about 0.6 or less,about 0.5 or less, or about 0.4 or less, and down to about 0.1. In someembodiments, compacting the cementitious slurry includes reducing S_(w)to a range of about 0.1 to about 0.7, about 0.2 to about 0.6, about 0.2to about 0.5, or about 0.2 to about 0.4. In some embodiments, compactingthe cementitious slurry is performed at a pressure in a range of about0.5 MPa to about 50 MPa, e.g., about 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40,45, or 50 MPa.

In some embodiments, exposing the structural component to carbon dioxideis performed at a temperature in a range of about 20° C. to about 85°C., about 30° C. to about 75° C., about 35° C. to about 70° C., or about40° C. to about 65° C.

In some embodiments, the low-carbon concrete product have up to 75%lower carbon intensity than a traditional cement-based concrete product.In some embodiments, the lower carbon intensity is due to (a) partialsubstitution of cement with portlandite and fly ash and/or (b) CO₂uptake during manufacturing. As understood by the skilled artisan, atraditional cement-based concrete product can have a carbon intensity ofabout 0.5 to about 1.5 tons of CO₂ per ton of OPC used in concreteproducts, e.g., about 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, or1.5 tons of CO₂ per ton of OPC used in concrete products. For example, atraditional cement-based concrete, and its products can have a carbonintensity of about 195 to about 771 kg CO₂e per m³.

It will be understood that in some embodiments other benefits or aspectsdisclosed more specifically below are also applicable to theabove-disclosed embodiments.

In another aspect according to some embodiments, a manufacturing processof a low-carbon concrete product includes: (1) providing a targetcompressive strength of the concrete product; (2) providing a predictionmodel relating carbon dioxide uptake to compressive strength; (3)forming a cementitious slurry including portlandite; (4) forming thecementitious slurry into a structural component; and (5) exposing thestructural component to carbon dioxide, thereby forming the low-carbonconcrete product, wherein exposing the structural component to carbondioxide includes monitoring carbon dioxide uptake of the structuralcomponent, and exposing the structural component to carbon dioxide isperformed at least until the carbon dioxide uptake of the structuralcomponent is indicative of meeting the target compressive strengthaccording to the prediction model.

In some embodiments, the carbon dioxide is contained within a CO₂ wastestream, such as a post-combustion or post-calcination flue gas stream,such as those described elsewhere herein.

Specific Embodiments and Examples

The following embodiments and examples describes specific aspects ofsome embodiments of this disclosure to illustrate and provide adescription for those of ordinary skill in the art. These embodimentsand examples should not be construed as limiting this disclosure, as theembodiments and examples merely provides specific methodology useful inunderstanding and practicing some embodiments of this disclosure.

Overview

In certain embodiments, a cementation solution is mineral carbonation(CO₂ mineralization), which is the reaction of CO₂ with inorganicprecursors to produce stable carbonate solids. Such reactions can beexploited to produce cement-replacement materials while sequestering CO₂from industrial emissions streams. To achieve cementation bycarbonation, a shape-stabilized “green-body” (e.g., block, slab, beam,and so forth) is exposed to fluid CO₂ (e.g., gas or liquid). Such insitu CO₂ mineralization is a multi-stage process that typically proceedsvia dissolution-precipitation (rather than direct solid-gas reaction),namely in some embodiments, the multi-stage process can proceed via thefollowing stages (for calcium-bearing reactants):

-   -   1) Dissolution of reactants to yield Ca²⁺ within liquid water in        pore network/water films at reactant surfaces,    -   2) Transport of CO₂ through the green body's pore network        towards pore water,    -   3) Dissolution of CO₂ in pore water and speciation to HCO₃ ⁻ or        CO₃ ²⁻, and    -   4) Reaction of dissolved species to precipitate mineral        carbonates (e.g., CaCO₃).

In addition to CO₂ sequestered into solid products, the embodied CO₂emissions of carbonating binders may be reduced vis-à-vis OPC bydiminishing production and use of OPC. This is because the reactants canbe industrial wastes (e.g., coal fly ash) and/or phases produced vialower-temperature routes (e.g., portlandite or Ca(OH)₂). CO₂mineralization of portlandite with flue gas is thermodynamically favoredat near-ambient temperatures. The final carbonation conversion ofportlandite particulates is found to be controlled by the relativehumidity RH of the contacting gas stream—i.e., independent oftemperature T and CO₂ concentration [CO₂] (FIG. 1 ). The weak dependenceof portlandite particulate carbonation to CO₂ concentration suggeststhat substantially enhancement of the CO₂ concentration beyond thattypical to flue gases, e.g., by membrane enrichment, is unlikely toyield proportional increases in the reaction kinetics of portlanditeparticles. This finding highlights the suitability of portlanditecarbonation within process cycles that use (un-enriched) post-combustiongas streams, which may be secured from natural gas or coal-fired powergeneration systems.

For readily-dissolved reactants such as portlandite, a constraint oncarbonation rates is CO₂ transport, which depends, in part, on thepresence of liquid water in the green body. Liquid water in porenetworks retards CO₂ transport by physical hindrance, since CO₂diffusion in water is about 10⁵ times slower than in air. Thestrengthening of binders containing rapidly dissolved Ca-bearing phasesalong with fly ash and OPC is a complex process, as strengthening isinduced by precipitation of both carbonates and of C-S-H (formed by OPChydration and pozzolanic reactions of Ca(OH)₂ with aluminosilicatesources such as fly ash).

Carbonation of Wet-Cast Compositions:

Investigation is made of the carbonation kinetics of mortars containingportlandite-enriched binders in contact with flue gas simulating thatfrom a coal fired power plant (about 12% CO₂). A representative bindercomposition includes about 42 mass % portlandite, about 25 mass % ASTMC618-compliant Class F fly ash (FA), and about 33 mass % OPC. Thisbinder is mixed with fine aggregate (sand) and water to form a mortar.The carbonation kinetics of these mortars is investigated as a functionof their initial pore saturation with water (S_(w)), which is controlledby drying prior to carbonation (FIG. 2A). The extent of CO₂ uptakeachievable within feasible processing durations increases significantlyas S_(w) is reduced, until saturation is reduced below a critical value(S_(w,c)). The value of S_(w,c) is between about 0.13 and about 0.07 forwet-cast compositions, indicating that this suppression of CO₂ uptake isdue to the reduction of water below the intrinsic level for Ca(OH)₂dissolution and carbonation product precipitation. Until this point,carbonation kinetics are hastened by the reduction of pore saturation,and are sufficiently rapid, even in direct exposure to diluted gasstreams, to render sufficient CO₂ uptake within a feasible processingduration. The strengthening resulting from carbonation (e.g., per unitCO₂ uptake) is described in FIG. 2B. Despite their varying CO₂ uptake,all compositions demonstrated broadly corresponding compressive strengthdevelopment, which is comparable or superior to that achieved solely bycement hydration (e.g., sealed curing) by the end of the processingperiod.

Concrete mixtures are typically specified to achieve design compressivestrength criteria at an age of 28 days after casting. To evaluate thecontinued strength development (and progress of hydration and pozzolanicreactions) in carbonated portlandite-enriched binders, mortar specimensare cured in saturated limewater following carbonation, and theircompressive strengths are measured (FIG. 3A). As a point of reference,similar evaluations are performed for non-carbonated specimens, andspecimens featuring a portlandite-free binder (about 75% OPC and about25% Class F fly ash). Despite its initially lower strength development,the carbonated portlandite-enriched binder achieved correspondingcompressive strength to that of the carbonated mixtures withoutportlandite, while achieving over about 4× greater CO₂ uptake. Thecontinued strength development is attributed to the continued progressof pozzolanic reactions (e.g., between residual calcium hydroxide andsilica-rich fly ash) and cement hydration during limewater curing. FIG.3B displays the mass fraction of calcium hydroxide in each specimen as afunction of specimen age, as assessed by thermogravimetric analysis. Theinitially elevated calcium hydroxide content of the portlandite-enrichedbinder is rapidly decreased with carbonation, and then shows a continualreduction due to consumption by pozzolanic reactions, at a higher ratethan the reference binder. The pozzolanic reaction manifested in thecontinued development of C-S-H, as indicated by the non-evaporable water(w_(n)) evolution, which was normalized by the OPC mass fraction (FIG.3C). This figure indicates that carbonated portlandite-enriched bindersshow a similar evolution of hydrated phases to the non-carbonatedreference OPC binders. This trend ensures that unlike the carbonated OPCbinder which demonstrates suppressed hydration/pozzolanic reactions intime, carbonated portlandite-enriched compositions continue to gainstrength at a higher rate due to formation of hydrated phases.

Carbonation of Dry-Cast Compositions:

Mortar formulations containing the same portlandite-enriched bindercomposition, but with elevated sand content and reduced water contentare also developed. Rather than being poured into a mold, thesespecimens are “dry-cast” into a mold and compacted using a hydraulicpress to become shape-stable, as for concrete masonry products. FIG. 4Acompares the CO₂ uptake of both wet-cast and dry-cast compositions as afunction of their saturation. While dry-cast components exhibit higherspecific CO₂ uptake than wet-cast specimens, both formulations exhibitsimilar trends with respect to pore saturation. The critical saturationS_(w,c) is similar between the two formulations within the experimentalresolution. This trend is also observed in dry-cast pellets composed ofsolely portlandite (FIG. 4B), indicating that the S_(w,c) reflects theintrinsic sensitivity of Ca(OH)₂ carbonation to proximate relativehumidity (RH). The strengthening of dry-cast specimens with increasingcarbonation durations is illustrated in FIG. 5A, which plots compressivestrength as a function of the relative density, a measure of the volumefraction of porosity within the specimens. Carbonation significantlyelevates compressive strength relative to non-carbonated samples, and isthe dominant contribution to strength development, for all relativedensities. At either duration of carbonation, the increase in strengthdue to carbonation is also approximately constant regardless of therelative density. This finding allows the carbonated strength to bepredicted from early age measurements of compressive strength ofnon-carbonated specimens.

The influences of reaction temperature on the carbonation kinetics andstrength development of dry-cast binders are also of note. Given thatincreasing temperature accelerates both the rate of drying and rate ofcarbonation, the effects of carbonation temperature on dry-cast mortarspecimens are evaluated as a function of the reaction temperature,without drying prior to CO₂ exposure (FIG. 5B). Increasing the reactiontemperature up to about 65° C. increased both the CO₂ uptake and 24-hcompressive strength substantially. However, further increasing thetemperature to about 85° C. diminished both CO₂ uptake and strength gainon account of the insufficient availability of water (S_(w)=0.06following CO₂ exposure at about 85° C. for about 12 h) to supportcarbonation reactions. This information indicates that carbonationprocessing may be applied without pre-drying, which allows enhancedprocess flexibility and increased throughput of CO₂ utilization. Thecritical saturation S_(w,c) observed previously (in cases in whichdrying did not appreciably reduce the saturation during carbonation)also holds when carbonation processing conditions yield simultaneousdrying (saturation reduction).

CO₂ Uptake—Strength Correlations:

To provide unifying guidelines describing the effect of carbonation onstrength development, FIG. 6 shows the carbonation strengthening factor(the ratio between carbonated and non-carbonated specimens) as afunction of CO₂ uptake. Both wet-cast and dry-cast compositions follow alinear trend of increasing carbonation strengthening with CO₂ uptake, upto a value of about 3.75. This allows forecasting of compressivestrength development resulting from various compositions and processing,provided that the CO₂ uptake is determined. This is important, as theCO₂ uptake can be assessed in real-time during carbonation, usingon-line instrumentation to quantify reductions in gaseous CO₂concentrations (e.g., nondispersive infrared (NDIR) sensor or gaschromatography).

Fulfillment of Strength Criteria:

Fulfilling design strength criteria (typically 1 day and 28-daystrengths) may be achieved via three primary levers: (1) changing thewater-to-binder mass ratio (w/b), (2) adjusting the mass proportions ofa ternary blend of portlandite-fly ash-OPC in the binder, and (3)altering the processing conditions. A strategy for fulfillingperformance criteria (e.g., strength) involves (i) implementing dryingprior to carbonation to adjust liquid water saturation in pores, (ii)elevating the temperature used during carbonation processing tosimultaneously enhance reaction kinetics and CO₂ transport properties(e.g., up to about 65° C.), (iii) reducing the water-to-binder massratio (w/b) to reduce volume of porosity, and (iv) increasing OPCcontent in binder system (e.g., at most ≤about 50 mass % of OPC). As anexample in FIG. 7 , it is highlighted that processing of a wet-castmortar by drying at T=about 45° C. for about 12 h resulted in a higherCO₂ uptake after carbonation for about 24 h, and also in a greaterstrength as compared to a similar mortar that was directly carbonated(without an initial drying stage) for a longer duration of about 36 h.After carbonation, the strength continued to increase during limewatercuring at which strength on the order of about 35 MPa was produced at 28days. Furthermore, by comparing strength results between FIG. 3A andFIG. 7 , for a similar processing condition, reducing w/b from about0.45 to about 0.40 and increasing OPC content from about 35% to about50% enhanced the 28-day strength from about 25 MPa to about 35 MPa.These findings demonstrate that adjustment of processing conditions andmixture proportioning can be implemented to develop carbonate-cementedsolids that take up CO₂ and provide strengths sufficient to fulfillstructural construction criteria (e.g., ≥about 30 MPa as per ACI 318;and ≥about 15 MPa as per ASTM C90 for concrete masonry units), asindicated in FIG. 5B and FIG. 7 .

Example Carbonation Processing and Strength Evolution ofPortlandite-Based Cementing Binders Overview

Binders containing portlandite (Ca(OH)₂) can take up carbon dioxide(CO₂) from dilute flue gas streams (<15% CO₂, v/v) thereby formingcarbonate compounds with binding attributes. While the carbonation ofportlandite particulates is straightforward, it remains unclear how CO₂transport into monoliths is affected by microstructure and pore moisturecontent. Therefore, this study elucidates the influences of poresaturation and CO₂ diffusivity on the carbonation kinetics and strengthevolution of portlandite-enriched composites (“mortars”). To assess theinfluences of microstructure, composites hydrated to different extentsand conditioned to different pore saturation levels (S_(w)) were exposedto dilute CO₂. First, reducing saturation increases the gas diffusivity,and carbonation kinetics, so long as saturation exceeds a critical value(S_(w,c)≈0.10); independent of microstructural attributes. Second,careful analysis reveals that both traditional cement hydration andcarbonation offer similar levels of strengthening, the magnitude ofwhich can be estimated from the extent of each reaction. As a result,portlandite-enriched binders offer cementation performance that issimilar to traditional materials while offering an embodied CO₂footprint that is more than 50% smaller. These insights are foundationalto create new “low-CO₂” cementation agents via in situ CO₂mineralization (utilization) using dilute CO₂ waste streams.

Cementation enabled by in situ carbonation is a promising alternative toconventional concrete that relies upon the reaction of CO₂ with alkalineinorganic precursors to precipitate carbonate solids. In this method, ashape-stabilized green body (e.g., block, slab, beam) is exposed to CO₂,e.g., in the gas, liquid, or supercritical states, which may be sourcedfrom CO₂ waste streams. Here, green bodies may be produced by eitherwet-casting (wherein a slurry is poured into a mold until it hardens andbecomes self-supporting) or dry-casting (in which components having verylow water contents are mechanically compacted until they areself-supporting). In the absence of water, the carbonation of mineralreactants such as portlandite (Ca(OH)₂) may proceed via gas-solidreaction. However, faster rates and greater extents of portlanditeconversion and CO₂ mineralization are realized when the presence ofliquid water promotes a dissolution-precipitation mechanism ofcarbonation, which entails the following steps for green bodies composedof calcium-bearing reactants:

-   -   The dissolution of the reactants releases Ca²⁺ species within        the pore liquid,    -   The dissolution and transport of CO₂ (i.e., as a gas/vapor or        dissolved carbonate ions) occurs from the outside environment        through the green body's pore network, and,    -   The reaction of dissolved species precipitates carbonate        minerals (e.g., CaCO₃).

The embodied CO₂ intensity of the resulting carbonated binder may besubstantially reduced vis-à-vis OPC depending on the nature of reactantsused. This is attributed to: (i) the direct sequestration of CO₂ from anemissions stream which fulfills the premise of CO₂ utilization, and (ii)the CO₂ avoidance associated with the substitution of OPC by industrialwastes (e.g., coal fly ash) or alkaline solids that may be produced by alow-temperature pathway, e.g., portlandite.

In green bodies composed using readily-dissolving reactants such asportlandite, CO₂ transport through the body is often the rate limitingstep in carbonation. In the absence of significant pressure gradients,CO₂ transport is dominated by diffusion. As the diffusivity of dissolvedCO₂ through water is ≈10⁴ times lower than that of gaseous CO₂ in air,the provision of air-filled porosity within green bodies is critical toaccelerating the rate of carbonation. The effective diffusivity ofpartially saturated pore networks is inversely proportional to themicrostructural resistance factor f(S_(w), ϕ). The microstructuralresistance to diffusion increases as the total porosity, ϕ, is reducedand as the volume fraction of porosity that is saturated with liquidwater, S_(w), is increased. The total porosity of portlandite-enrichedcomposites is a function of their composition (e.g., water-to-bindermass ratio, aggregate content), method of forming (e.g., wet-cast vs.dry-cast, and degree of consolidation), and the extent of hydration andcarbonation reactions that may have occurred. On the other hand, S_(w)can be reduced by using dry-cast mixtures with low water contents, or bydrying before (or during) CO₂ exposure. However, large reductions inS_(w) may depress the internal relative humidity (RH) within the greenbody's pores; a relationship which is described by the material's watervapor sorption isotherms. This is significant, as the RH of theCO₂-containing gas stream (“reaction environment”) that is contactingportlandite has been noted to significantly impact its carbonationbehavior. For example, portlandite's carbonation in dry conditions(RH≈0%) is hindered (e.g., less than 10% conversion), due to surfacepassivation associated with gas-solid carbonation. Increasing the RH isnoted to promote a dissolution-precipitation pathway, which enables nearcomplete conversion (e.g., in excess of 80%). Although the important ofthe reaction environment's RH on the carbonation of portlanditeparticulates is recognized, the effect of pore saturation on thecarbonation of portlandite-based monoliths remains unclear.

The fabrication of carbonated wet-cast or dry-cast structural concretecomponents that fulfill specific engineering performance criteriarequires a detailed understanding of the mechanisms of cementation(strengthening) therein. Although it is known that the products ofcarbonation, OPC hydration, and pozzolanic reactions can adhereproximate surfaces and induce reductions in porosity, the contributionsof these reactions to strength gain, especially in carbonatedcomposites, remain unclear. For example, during CO₂ exposure, thesereactions occur concurrently, making it difficult to isolate thecontributions of each reaction to strength gain. Furthermore, C-S-Hprecipitation on reactant surfaces and within pore spaces, prior tocarbonation, may limit strengthening by hindering CO₂ diffusion andreducing the availability of exposed reactant (portlandite) surfaces.Finally, it is unknown whether conventional relationships between theextent of hydration and strength hold true during CO₂ exposure, asprocessing conditions that may favor carbonation (e.g., decreasing S_(w)by drying) may suppress OPC hydration and pozzolanic reactions due tothe consumption of portlandite. To overcome gaps in knowledge toimplement carbonation-based cementation, this example primarily aims toelucidate the influences of microstructure on the carbonation kineticsof portlandite-enriched cementing composites (“mortars”). The premise ofusing portlandite is straightforward for a multiplicity of reasonsincluding:

-   -   Making use of existing facilities: Portlandite can be produced        using limestone as a precursor using existing OPC kilns and        features a cost that is essentially similar to OPC,    -   Lower processing temperature: Portlandite's production, by the        decarbonation of limestone around 800° C. (at ambient pressure,        in air), followed by the hydration of lime requires a processing        temperature that is nearly 700° C. lower than OPC production,    -   Straightforward carbonation: Unlike OPC and other potential        alkaline precursors, portlandite carbonation is only slightly        affected by temperature and CO₂ partial pressure for conditions        relevant to flue gas exposure (≈4-15% CO₂, v/v), provided that        the RH of the contacting gas is sufficient to promote liquid        water-mediated carbonation, and,    -   Highest CO₂ uptake: Due to its substantial calcium content,        portlandite features among the highest potential CO₂ uptake (59        mass %) of mineral reactants that may be achieved in contact        with flue gases. For example, although Mg(OH)₂ has a higher        potential CO₂ uptake (75 mass %), it requires a greatly elevated        temperature and pressure to achieve similar carbonation kinetics        (rates) as portlandite.

Taken together, the findings highlight that portlandite-enriched binderscan serve as a viable functional replacement for OPC-based cementationagents, and offer new insights to design concrete constructioncomponents that are cemented via in situ CO₂ mineralization.

Materials and Methods: Materials and Sample Preparation

Portlandite-enriched binders were composed of: 42 mass % portlandite, 33mass % ASTM C150-compliant ordinary portland cement (Type II/V OPC) and25 mass % ASTM C618-compliant Class F fly ash (FA). OPC was incorporatedto provide green strength and to facilitate handling prior to drying andcarbonation, whereas FA served as a source of aluminosilicates topromote pozzolanic reactions. A portlandite-free reference binder (i.e.,75 mass % OPC and 25 mass % FA) was also formulated to isolateportlandite's influences on reactions and strength evolution. Theportlandite (Mississippi Lime) used featured a purity of 94%±2% (bymass) with the remainder being composed of CaCO₃ as determined bythermogravimetric analysis (TGA). The median particle diameters (d₅₀) ofportlandite, FA, and OPC were 3.8 μm, 8.9 μm, and 17.2 μm, respectively,as determined using static light scattering (SLS; LS13-320, BeckmanCoulter). Further details on the chemical composition and particle sizedistributions of binder solids are reported in the SupportingInformation (SI).

The binders were combined with ASTM C33 compliant silica sand (fineaggregate) to form composites (“mortars”) as described in ASTM C305.Wet-cast composites were formulated at w/b=0.45 (w/b=water-to-bindermass ratio) and a/b=3.5 (a/b=aggregate-to-binder mass ratio). Dry-castcomposites had w/b=0.25 and a/b=7.95. The fine aggregate had a densityof 2650 kg/m³ and a water absorption of ≤1.0 mass %. Acommercially-available polycarboxylate ether (PCE) dispersant was addedto enhance the fluidity of the wet-cast composites at a dosage of 0.8%of the binder mass. The wet-cast composites were molded into cylinders(50 mm×100 mm; d×h) and vibrated to remove entrapped air. Dry-castcomposites were prepared by compaction using a hydraulic press to formcylindrical specimens (75 mm×40 mm; d×h) that featured a surfacearea-to-volume ratio (SA/V, mm⁻¹) equivalent to the wet-cast specimens.The compaction pressure was varied between 0.5 MPa and 22.0 MPa toachieve relative densities (ρ/ρ_(s), the ratio of bulk density toskeletal density) ranging between 0.58-to-0.88. Dry-cast portlanditecomposites with w/b=0.25 and a/b=7.95 as well as neat portlanditepellets (10 mm×8 mm; d×h) with different water-to-solid (i.e.,portlandite) mass ratios between 0 and 0.75 were also formed bycompaction for comparative analyses.

Drying and Carbonation Processing

The wet-cast composites were cured under sealed conditions for 6 h atT=22±2° C. to achieve shape stability and a compressive strengthσ_(c)≈0.5 MPa. The specimens were then either carbonated immediatelyafter forming or dried under exposure to flowing air to achievedifferent initial S_(w) prior to carbonation. In contrast to thewet-cast composites, the initial S_(w) of the dry-cast composites wasaltered by applying different compaction pressures. During drying andcarbonation, the cylindrical specimens were placed in custom-builtreactors with an internal diameter of 100 mm and a length of 150 mm (seeschematic, FIG. 14 in SI). The reactors were placed in an oven fortemperature regulation and the flow rate of the inlet gas was controlledby mass-flow controllers. Different drying conditions were implementedby varying the: (i) air temperature (22±0.5° C., 45±0.5° C., and 65±0.5°C.), (ii) air flow rate (0.5 slpm to 40 slpm; standard liters perminute), and, (iii) drying duration (0 to 12 h). Carbonation during airdrying was very limited. The average CO₂ uptake of wet-cast specimensprior to CO₂ exposure was 0.015±0.005 g_(CO2)/g_(reactants). The driedspecimens were then contacted with simulated flue gas at a flow rate of0.5 slpm for up to 60 h at different isothermal temperatures (22±0.5°C., 45±0.5° C., and 65±0.5° C.). The simulated flue gas was prepared bymixing air and CO₂ to mimic the exhaust of a coal power plant. The CO₂concentration of the gas was 12±0.2% [v/v] as confirmed using gaschromatography (GC; F0818, Inficon).

Experimental Methods

Time-dependent CO₂ uptake was quantified using thermogravimetricanalysis (TGA: STA 6000, Perkin Elmer). The values reported are theaverage CO₂ uptake of three powdered samples taken along the height ofthe cylindrical specimens. Around 30 mg of each powder was placed inpure aluminum oxide crucibles and heated at a rate of 15° C./min over atemperature range of 35° C. to 975° C. under UHP-N₂ gas purge at a flowrate of 20 mL/min. The CO₂ uptake was quantified as the mass lossassociated with CaCO₃ decomposition over the temperature range of 550°C. to 900° C., normalized by the total mass of solids in the binder(i.e., portlandite, fly ash, and OPC). Towards this end, the mass lossassociated with CaCO₃ was initially normalized by the total sample mass(i.e., aggregate+binder solids) in the form of g_(CO2)/g_(solid). Theresults were then normalized by the fraction of binder present in thetotal solids (i.e.,g_(CO2)/g_(solid)*g_(solid)/g_(reactants)=g_(CO2)/g_(reactants)), whichwas determined from the mixture proportions. It should be noted that theinitial CO₂ content (i.e., carbonate minerals within the aggregates andbinder) and the CO₂ uptake during drying were subtracted from theoverall CO₂ uptake measured during carbonation, to eliminate theirinfluences on the experimental results. The non-evaporable water content(w_(n), mass %) was calculated as the mass loss over the temperaturerange of 105° C. to 975° C. excluding the mass loss from thedecomposition of CaCO₃ and Ca(OH)₂.

The compressive strength of the composites was measured as per ASTM C39.Appropriate strength correction factors were applied in consideration ofthe specimens' length-to-diameter ratios to allow direct comparisonsbetween the dry-cast and wet-cast specimens, which feature slightlydifferent geometries.

The total porosity and pore (moisture) saturation level of thecomposites before and after carbonation were quantified using a vacuumsaturation method. Cross-sectional disks, 25 mm-thick were sectionedfrom the middle of the cylindrical specimens using a low-speed saw.Isopropanol (IPA) was used as the solvent to arrest hydration. The CO₂diffusivity was estimated from the total moisture diffusion coefficient,D_(wt) (m²/s), (i.e., the sum of liquid water and water vapor diffusioncoefficients) of the composites prior to CO₂ exposure using Fick's2^(nd) law of diffusion, as elaborated in the SI.

Results Influences of Saturation on CO₂ Uptake in Portlandite-EnrichedBinders

The carbonation kinetics of wet-cast composites pre-dried to differentinitial S_(w) (FIG. 15 in SI) were evaluated. FIG. 8A displays thetime-dependent CO₂ uptake of each specimen C(t) normalized by the massof reactants, i.e., portlandite, fly ash, and OPC. The measured data wasfitted to an equation of the form C(t)=C (t_(u)) [1−exp((−kt)/C(t_(u)))] to estimate the apparent carbonation rate constant (k,h⁻¹), and C(t_(u)), the ultimate CO₂ uptake, where t_(u) is taken as 60h. Reducing S_(w) from 1.00 (complete saturation) to 0.13 for thewet-cast composites increased both the carbonation rate constant and theultimate CO₂ uptake by nearly 10× (FIG. 8A). The same observation istrue for dry-cast composites, demonstrating the significance of S_(w) asa controlling variable on carbonation kinetics across different formingmethods and microstructures. But, enhanced levels of CO₂ uptake wereobtained for the dry-cast relative to wet-cast composites at comparableS_(w) (FIG. 8B), as further discussed below.

The carbonation of both the wet-cast composites and neat portlanditecompacts was nearly fully suppressed when S_(w) was reduced below acritical value, S_(w,c)≈0.10 (FIG. 8C). It should be clarified thatalthough the trend fitted to the data for dry-cast composites indicatesa higher critical saturation S_(w,c)=0.30 (identified as S_(w) at whichthere is a maximum in CO₂ uptake), this estimation resulted from a lackof data corresponding to dry-cast composites within 0.03<S_(w)<0.30.However, separate data obtained for the neat portlandite compacts (FIG.8C) indicated a critical saturation level, S_(w,c)=0.14. This value issimilar to S_(w,c)=0.12 that was determined for the wet-cast composites(FIG. 8C). This implies that S_(w,c)≈0.10 is an intrinsic limit onportlandite carbonation, which is sustained for composites prepared byboth wet-cast and dry-cast forming methods. Assuming that the watervapor sorption isotherms of portlandite-enriched binders arefunctionally similar to those of typical cementitious binders,S_(w,c)≈0.10 corresponds to an internal RH≈10%. These findings broadlyagree with the minimum ambient RH_(c)=8% to promote carbonationreactions via a dissolution-precipitation pathway. So long as S_(w,c) isexceeded, Ca²⁺ species liberated following the dissolution ofportlandite and other Ca− bearing reactants (OPC and fly ash) react withdissolved CO₂ species (i.e., CO₃ ²⁻ and HCO₃ ⁻) to precipitate calciumcarbonate. However, below S_(w,c), carbonation may be hindered by thereduced mobility and availability of water to support dissolution andprecipitation; thus, carbonation should, in some embodiments, proceed bygas-solid reaction, which is limited by surface passivation. Thisobservation suggests that carbonation suppression at low S_(w) mayresult from a shift in the reaction mechanism, which is applicableacross processing and preparation conditions. Therefore, maintainingS_(w)>S_(w,c) is an important requirement in some aspects for thecarbonation of portlandite-enriched binders to enhance CO₂ uptake andthe carbonation strengthening. A detailed analysis of the TGA traces ofportlandite-enriched composites indicated that Ca(OH)₂ was rapidlyconverted upon CO₂ exposure, and accounted for nearly all of the overallCO₂ uptake within the first 10 h CO₂ exposure. As portlandite conversionslowed, the contribution of other solid such as C-S-H becamesignificant. These results suggest that the overall carbonation ratelargely corresponded to that of portlandite carbonation initially, witha progressive switchover, in time, to the carbonation of other solidphases which including C-S-H. The contributions of other solid phases tothe total CO₂ uptake of binder for the wet- and dry-cast compositesranged between 2%-15% and 20%-38%, respectively, after 60 h CO₂exposure. These results indicate that the overall CO₂ uptake ofportlandite-enriched composites is largely dominated by portlanditecarbonation.

The differences in the carbonation kinetics between wet-cast anddry-cast composites are on account of the composites' microstructuralresistances to CO₂ diffusion. Here, the CO₂ diffusivity was indirectlyestimated by the total moisture diffusivity, which was measured byone-dimensional drying experiments. Although the mechanisms by which CO₂and moisture (i.e., in the form of liquid and vapor phases) diffusethrough pore networks may somewhat differ, they are both controlled bythe total porosity, tortuosity and saturation level of the porestructure. The total moisture diffusivities of the composites wereestimated at the time immediately prior to the initiation ofcarbonation. At equivalent S_(w), the dry-cast composites showed ahigher moisture diffusivity than wet-cast composites, due to their lowerdegree of OPC hydration (FIG. 9A). This reinforces the premise thatmicrostructural resistance controls CO₂ diffusion and carbonationreaction kinetics. The carbonation rate constant of wet-cast anddry-cast composites at varying S_(w) shows a similar logarithmic scalingas a function of the total moisture diffusivity for S_(w)≥0.13 (FIG.9B). It should be noted, however, that the dry-cast composites showedrate constants that are systematically higher than those of wet-castcomposites for equivalent diffusivities. This difference is postulatedto result from the different extents of OPC hydration of the twocomposites, as reflected in their non-evaporable water contents (FIG.9C). Indeed, the CO₂ uptake of both wet-cast and dry-cast compositesdecreased at a similar rate with increasing non-evaporable watercontent. The enhanced carbonation kinetics of the dry-cast composites istherefore consistent with the elevated accessibility of portlanditesurfaces therein, due to such surfaces being less occluded by C-S-Hprecipitates which may impose transport barriers to CO₂ contact andintrusion. This indicates that if OPC hydration in wet-cast compositeswas limited to a degree similar to that of the dry-cast composites(while ensuring shape stability) they too may feature enhanced CO₂uptake.

Carbonation Strengthening of Portlandite-Enriched Binders

The compressive strengths of the portlandite-enriched compositesincreased over the course of CO₂ exposure due to carbonation and OPChydration (FIG. 10A). Notably, despite their lower extents of OPChydration (i.e., w_(n)/m_(OPC)), the carbonated composites featuredstrengths equivalent to or greater than that of a sealed composite,i.e., in which OPC was permitted to hydrate without CO₂ exposure.Strength slightly increased as the initial S_(w) was reduced, owing tothe increased CO₂ uptake (FIG. 16 in SI). However this was so only aslong as S_(w)>S_(w,c), because, in general, both carbonation andhydration are suppressed at low internal RH. The critical poresaturation required to sustain OPC hydration is substantially higherthan that of carbonation reactions. For instance, the hydration of alite(Ca₃SiO₅, the major phase in OPC) is suppressed when the internal RHdrops below 80%. The dry-cast composites showed a contrasting trend,whereby strength increased with S_(w) (FIG. 10 b B). However, this is,in part an artifact resulting from the reduction in total porosity thatresulted from the increased levels of compaction that were used toelevate S_(w). For example, analytical analysis of particle packingwithin the dry-cast composites reveals a 4× reduction in theinterparticle spacing as the relative density increased from 0.67 to0.88. Not only does this improve particle-to-particle contacts, but italso permits more effective cohesion in the material by a smallerquantity of cementing agent (carbonate precipitates).

Unlike carbonated pastes composed only of fly ash, the strength-CO₂uptake curves of portlandite-enriched composites with different initialS_(w) cannot all be fitted by a single linear relation, i.e., with ashared slope m=Δσ_(c)/ΔC; MPa/(g_(CO2)/g_(reactant)) that remainsconstant over the course of carbonation (see FIG. 10 ). Rather, bothwet-cast and dry-cast composites demonstrated unique bi-linear trendswherein the secondary slope m₂ (i.e., between t=6 h and t=60 h) wassteeper than the initial slope m₁ (i.e., between t=0 h and t=6 h); i.e.,indicating an increase in the strength gain per unit CO₂ uptake at laterages after the cementing agent first cohered the solid skeletontogether. Interestingly, the later-age slope m₂ increased exponentiallywith the normalized non-evaporable water content Δ(w_(n)/m_(OPC)) andeventually sketched a single curve for both wet-cast and dry castmixtures (FIG. 10C). As such, extrapolation to Δ(w_(n)/m_(OPC))=0 (i.e.,when the OPC would remain unreacted during CO₂ exposure) yields ay-intercept of 14.7 MPa per unit mass CO₂ uptake(g_(CO2)/g_(reactants)). This value reflects the strength gain per unitmass of CO₂ uptake in the portlandite-enriched binder in the absence ofconcurrent strengthening by OPC hydration. This level of strengtheningis substantially higher than the 3.2 MPa per unit mass of CO₂ uptakenoted for fly ash reactants (g_(CO2)/g_(fly ash))—an unsurprisingoutcome given the much higher mobility and availability of Ca-speciesand greater carbonation reaction rate provisioned by portlandite.Similar analysis of the strength-w_(n)/m_(OPC) relation over the courseof CO₂ exposure (FIG. 10D) indicates that OPC hydration results instrength gain of ≈0.38 MPa per unit of OPC reacted (w_(n)/m_(OPC)). Assuch, assuming that the binding effects of carbonation and OPC hydrationare additive, for processing carried out at 22° C., strength developedcan be estimated by an equation of the formσ_(c)(t)=A·C(t)+B·w_(n)(t)/m_(OPC) where A=14.7MPa/(g_(CO2)/g_(reactants)) and B=0.38 MPa/(w_(n)/m_(OPC)) as determinedfrom the slopes of the strength-CO₂ uptake and strength-w_(n)/m_(OPC)curves.

Note, the strength gain per degree of OPC hydration estimated above issimilar to that observed during sealed curing in the absence of CO₂exposure (FIG. 16 in SI) and to that within portlandite-free composites,indicating that carbonation, and the presence of portlandite as areactant does not explicitly induce a change in the composition (i.e.,Ca/Si, molar ratio) or binding performance of the reaction products thatare formed. It should be noted however, that the strength predictionequation noted above offers better estimates for the dry-cast, asopposed to the wet-cast composites. This is on account of the effects ofdrying (FIG. 17 in SI). For example, unlike drying at a low flow rate of0.5 slpm (i.e., similar to that used for carbonation), increasing theflow rate to enhance drying depressed the rate of strength gain perdegree of OPC hydration. This may be attributed to the effects ofmicrocracking, and/or heterogeneity in microstructure with respect tothe nature of hydration products that may form resulting from theaccelerated extraction of water, especially at higher temperatures. Dueto the inherently lower water content of the dry-cast composites, andthe reduced extent of OPC hydration that results—dry-cast composites aretherefore less affected by processing conditions prior to carbonation.

Nevertheless, analysis of the carbonation strengthening factor (F_(cs),unitless), i.e., the ratio of the strength of carbonated tonon-carbonated composites revealed that dry-cast composites composed ofneat-portlandite achieved F_(cs)=3.75 (FIG. 11A). This was substantiallyhigher than the strengthening factors achieved for wet-cast composites(F_(cs)≤2.5) and dry-cast composites (F_(cs)≤3.25)—and confirms that thestrengthening offered by the in situ formation of carbonates isfoundational in ensuring cohesion and strength development (FIG. 18 inSI). Interestingly, F_(cs) of dry-cast mixtures was inversely correlatedto their relative density (ρ/ρ_(s)) indicating that the strengtheningeffect arising from compaction/particle interlock reduces the relativeinfluence of carbonation and the bridging action of cementingprecipitates.

S_(w), can be additionally controlled, especially in dry-castcomposites, by changing the temperature, i.e., by imposing drying usinga heated gas stream, prior to and during carbonation. As noted in FIG.11B, elevating the reaction temperature substantially enhanced both CO₂uptake and strength, resulting in the development of σ_(c)≈25 MPa in 24h. This is attributed to both facilitated CO₂ transport due to theremoval of water by evaporation (increased carbonation reaction rate),and the stimulation of OPC hydration and pozzolanic reactions (asindicated by w_(n)/m_(OPC) in FIG. 11B). However, in agreement with theresults for drying-induced changes in S_(w), a temperature increase isbeneficial to a limit—further increasing the temperature to 85° C.diminished both CO₂ uptake and strength gain on account of theinsufficiency of pore water to support both CO₂ mineralization and OPChydration reactions. This is attributed to: (a) the exothermic nature ofcarbonation reactions wherein temperature rise (unless the heat israpidly dissipated) shifts the reaction equilibrium towards thereactants thereby resulting in a retardation in reaction progress;following Le Chatelier's principle, and (b) the rapid extraction ofwater, as a result of which carbonation and hydration are bothsuppressed due to the rapid decrease in the liquid saturation level inthe pores. These observations suggest that use of a partially humidifiedCO₂ (flue gas) stream could favor carbonation in composites having lowwater contents (e.g., dry-cast composites) that are processed at highertemperatures. As an example, the flue gas emitted from a coal-firedpower plant features temperatures (T) and a water vapor contents (w_(v),v/v) on the order of 50° C.≤T≤140° C. and 12%≤w_(v)≤16%, respectively.The water (vapor) present in the flue gas could thus compensate forwater loss due to evaporation at such temperatures.

Long-Term Strength Development of Carbonated Composites

The strength evolution of carbonated composites following an initialperiod of CO₂ processing is relevant because the compressive strength ofcementitious materials at 28 days currently serves as an importantcriterion/specification/compliance attribute in structural design.Therefore, wet-cast portlandite-enriched composites with S_(w)=0.65 wereeither: (a) cured in saturated limewater (Ca(OH)₂ solution) at 22° C.for up to 28 d, or (b) carbonated for 12 h at 45° C. (in 12% CO₂, v/v)before curing in saturated limewater was continued until 28 d. To betterassess the effects of portlandite enrichment, the strength evolutions ofportlandite-free composites (i.e., where the binder was simply composedof OPC and FA) were also examined. In portlandite-free composites,carbonation induced a small increase in compressive strength and CO₂uptake at early ages (≈3% by mass of binder) relative to theportlandite-enriched composites at an equivalent carbonation reactiontime of 12 h. However, the rate of strength gain diminished over time(FIG. 12A) due to the coverage of reacting particle surfaces bycarbonate (and perhaps C-S-H) precipitates, which hinders hydration andpozzolanic reactions relative to non-carbonated, and non-portlanditeenriched composites in the longer term, i.e., see reduced non-evaporablewater contents as shown in FIG. 12B. As a comment of substance: thisdraws into question the approach of carbonating fresh OPC-basedcomposites with respect to late-age strengthening and durability. Forinstance, Zhang et al. reported that the carbonation of early-age OPCconcrete can result in formation of carbonate precipitates on C₃S/C₂Sparticles which can lead to suppression of strength at later ages duringpost-hydration. Furthermore, it has been noted that the reduced calciumhydroxide content of carbonated OPC-based concretes can increase therisk of corrosion of reinforcing steel. Similar reductions in thereactivity of OPC-based materials following carbonation have often beenattributed to the formation of surficial barriers (e.g., as alsorelevant for prehydrated cements) on anhydrous and/or hydrated OPCphases (C-S-H and Ca(OH)₂), and to the consumption of Ca(OH)₂ duringcarbonation (see FIG. 12C).

In contrast, portlandite-enriched composites exposed to CO₂ featuredstrengths that are higher than that of the non-carbonated referencecomposite not only during CO₂ exposure but also when cured in limewater,manifesting a strength that is nearly 7 MPa (≈40%) higher after 28 daysof aging (Figure A. OPC hydration in the carbonated portlandite-enrichedcomposites, interestingly, was suppressed to only a minor degreerelative to its non-carbonated reference (FIG. 12B) and was nearlyequivalent to that of the hydrated portlandite-free binder. This natureof enhanced later-age strength development of the portlandite-enrichedcomposites suggests that surface localization of carbonation products inthe vicinity of the easier to carbonate portlandite grains results inreduced surface obstructions on OPC (and other reactant) particulates inthese composites. Moreover, despite the significant consumption ofportlandite during carbonation, the progress of pozzolanic reactions ofcarbonated portlandite-enriched binders proceeded unabated duringcuring, as represented by the progressive increase in non-evaporablewater content (FIG. 12B) and the corresponding reduction in portlanditecontents (FIG. 12C). It is furthermore observed that despite substantialportlandite consumption in the carbonated portlandite-enrichedcomposite, residual portlandite remains that is not converted intoCaCO₃. While this does suggest the potential to extend the carbonationprocessing window (i.e., to consume more portlandite), it shows anability to explicitly control how much residual portlandite remains,e.g., to maintain a sufficient pH buffer to allow for the formation ofpassivation films on reinforcing steel surfaces as appropriate to hindercorrosion. Notably, the portlandite-enriched composite had an equivalent28 d strength to the carbonated portlandite-free composite, whilecontaining less than half of the OPC content and while taking up 4.3×more CO₂. Admittedly, this strength was around 83% that of the reference(non-carbonated) OPC-FA composite. However, the embodied CO₂ intensityof the carbonated portlandite-enriched composite is—conservatively,i.e., in spite of incomplete portlandite consumption—more than 50% lowerwhen aspects of both CO₂ avoidance and uptake are taken into account.Further, by applying a slightly higher temperature as typical for fluegas exhaust, it is noted that portlandite-enriched dry-cast compositeswere able to deliver the same strength as their wet-cast counterparts(FIG. 11 )—although in 24 h rather than 28 d, and once again, with agreatly reduced embodied CO₂ footprint.

This example has elucidated the potential of in situ CO₂ mineralizationand the formation of carbonate precipitates as a pathway for: (a)ensuring the cementation of construction relevant components, and (b) asa means for enabling the utilization of dilute CO₂ waste streams atambient pressure, and near-ambient temperatures with any need forpre-/post-treatment. The understanding gained offers new means to designlow-CO₂ cementation agents that can serve as a functional replacement toOPC, the very CO₂-intensive cementation agent used by the constructionsector for over a century. Special focus was paid to elucidate the rolesof microstructure and pore (moisture) saturation on affecting CO₂transport into 3D-monoliths, and the consequent impacts on the rate andprogress of carbonation reactions and strength development. In general,while reducing pore saturation enhances carbonation, this is only trueso long as S_(w,c)>0.10, below which the hindered dissolution ofportlandite, in turn, suppresses carbonation. Unsurprisingly, dry-castcomposites due to their lower water content, and the reduced surfacecoverage produced on their reactant surfaces (e.g., due to OPChydration) are more effectively carbonated. Importantly, it is shownthat the formation of carbonate precipitates is able to effectively bindproximate surfaces mineral particle surfaces thereby resulting in thecarbonated dry-cast composites that achieve a compressive strength of≈25 MPa in 24 h. It is furthermore shown that the formation of carbonateprecipitates yields strengthening at the level of ≈15 MPa per unit ofCO₂ uptake of reactants. This is substantially higher, e.g., than thatnoted by Wei et al. in their studies of fly ash carbonation. Theoutcomes of this work offer guidelines regarding process routes todevelop portlandite-enriched cementation agents. This is significant assuch novel binders, on account of their CO₂ uptake and avoidance,feature a CO₂ intensity that is substantially lower than that of typicalOPC-based binders, which are commonly, today, diluted using fly ash. Asan example, the global warming potential (GWP; kg CO₂e/m³) associatedwith production of raw materials, transportation, and manufacturing ofthe concrete masonry units (CMUs) indicate that representativeportlandite-enriched CMU formulations feature a GWP that is nearly 58%lower than that of typical OPC-dominant CMU formulations (Table S2 inSI). This GWP reduction is attributed to (i) the substitution of OPCwith portlandite and fly ash (CO₂ avoidance), and (ii) the net negativeCO₂ emissions associated with CO₂ uptake during manufacturing (CO₂utilization). Evidently, the nature of processing conditions discussedherein are well-suited for the precast manner of fabrication. Thiscreates opportunities to utilize portlandite-enriched binders tomanufacture both masonry and precast components that can be used forboth structural (“load bearing”) and non-structural construction.

Supporting Information: (A) Materials

The bulk oxide compositions of the ordinary portland cement (OPC) andfly ash as determined using X-ray fluorescence (XRF) are presented inTable 51. The densities of the portlandite, fly ash, and OPC weremeasured using helium pycnometry (Accupyc II 1340, Micromeritics) as:2235 kg/m³, 2460 kg/m³, and 3140 kg/m³, respectively. The particle sizedistributions (PSDs) of the binder solids were measured using staticlight scattering (SLS; LS13-320, Beckman Coulter; see FIG. 13 ).

TABLE S1 Oxide composition (by mass) of the fly ash and OPC asdetermined by XRF. Mass (%) Oxide Fly ash Type I/II OPC SiO₂ 60.84 21.21Al₂O₃ 22.30 4.16 Fe₂O₃ 4.75 3.85 SO₃ 0.62 2.81 CaO 6.38 65.50 Na₂O 2.070.18 MgO 1.80 1.98 K₂O 1.23 0.32

(B) Drying and Carbonation Processing

A schematic of the drying and carbonation reactors and related onlineinstrumentation is illustrated in FIG. 14 .

(C) Experimental Methods

Moisture diffusion coefficient: The sides of the cylinders (50 mm×25 mmfor wet-cast and 75 mm×25 mm for dry-cast; d×h) were sealed using asilicone sealant and aluminum tape to ensure 1-D diffusion. For thisboundary conditions, Fick's 2^(nd) law can be expressed analyticallyusing a Taylor expansion of the error function as follows:

$\begin{matrix}{\frac{m_{t}}{m_{\infty}} = {1 - {\sum\limits_{n = 0}^{n = \infty}{\frac{8}{\left( {{2n} + 1} \right)^{2}\pi^{2}}{\exp\left( \frac{{- {D_{tot}\left( {{2n} + 1} \right)}^{2}}\pi^{2}t}{4L^{2}} \right)}}}}} & {{Equation}({S1})}\end{matrix}$

where m_(t) (g) is the mass loss at a given time, m∞, (g) is theultimate mass loss (i.e., at the infinite time; at equilibrium), t (s)is time, and L (m)=0.0125 m is half of the sample thickness.

(D) Kinetics of Drying Prior to Carbonation

The effects of temperature and air flow rate on the drying kinetics ofwet-cast composites (“mortars”) and the reduction in the degree ofliquid saturation, S_(w), are shown in FIGS. 15A and B. Expectedly,higher temperatures or air flow rates accelerated drying and resulted ina prominent decrease in S_(w). S_(w) plateaued over time under alldrying conditions, and more rapidly so at higher temperatures. Thisplateau indicates a progressive transition in the size of pores fromwhich water is removed. Specifically, as the internal RH diminishes,water is first drawn out from larger and percolated pores, andthereafter smaller sub-micron and disconnected pores. FIG. 15C shows theeffect of compaction pressure on increasing S_(w) for dry-castcomposites that is induced by decreasing their total porosity.

(E) Carbonation Strengthening

FIG. 16 displays the evolution of compressive strength as a function ofthe non-evaporable water content, w_(n)/m_(OPC), for wet-cast compositesacross increasing carbonation durations. Significantly, the compressivestrengths developed in carbonated composites are equivalent or superiorto the sealed cured composites wherein, in the latter, strengthdevelopment is simply ensured by the hydration of OPC.

FIG. 17 displays the dependence of slope of strength—w_(n)/m_(OPC)relation on drying conditions.

FIGS. 18A and B compare the microstructure and surface morphology ofcarbonated wet-cast and dry-cast composites at varying degrees ofhydration. The images were acquired using a field emission-scanningelectron microscope with an energy dispersive X-ray spectroscopydetector (SEM-EDS; FEI NanoSEM 230). For a given time, cross-sectionaldisks were taken from the cylinders and immersed in IPA for 7 days tosuppress OPC hydration. The disks were then vacuum-dried in a desiccatorfor 7 days, before small coupons were taken from the disks andimpregnated with epoxy, polished, and gold-coated. All SEM micrographswere acquired in secondary electron mode with a spot size of 4.0 nm, atan accelerating voltage of 10 kV, and a working distance between ≈5.5mm.

(F) Representative Sustainability Implications/Assessments

The global warming potential (GWP; kg CO₂e/m³) of representativeportlandite-enriched concrete masonry units (CMUs) has been estimated inline with the Environmental Product Declaration (EPD) methodology andcompared with the OPC-based CMUs. For concrete masonry products, this isdescribed by the product category rule (PCR): “ASTM International. ASTMInternational PCR005: Product Category Rules for Preparing anEnvironmental Product Declaration for Manufactured Concrete and ConcreteMasonry Products, 2014; p 21.” EPDs following this PCR use the productstage as the system boundary, and therefore include three modules: (1)raw materials supply, (2) transport to the manufacturer, and (3)manufacturing. The declared unit is 1 m³ of concrete masonry products.Table S2 provides a comparative evaluation of the GWP of each module ofa Canadian industry-averaged EPD (representative of conventionalOPC-based CMU) against the GWP of a representative portlandite-enrichedbinder designed for CMU fabrication. This calculation indicates that theportlandite-enriched CMU features a GWP that is ≈58% less than that ofconventional OPC-based CMU. This reduction is attributed to (i) thesubstitution of OPC with portlandite and fly ash (CO₂ avoidance), and(ii) the net negative CO₂ emissions associated with CO₂ uptake duringmanufacturing (CO₂ utilization).

TABLE S2 The comparative global warming potential (GWP, kg CO₂e/m³) forportlandite- enriched and OPC-concrete masonry based on a cradle-to-gateanalysis. The declared unit is 1 m³ of concrete formed into masonryunits (CMUs) as per applicable product category rules. GWP of OPC-basedCMU GWP of Portlandite- (kg CO₂e/m³) [sourced from enriched CMU ModuleCCMPA average EPD] (kg CO₂e/m³) A1: Raw Material Supply 170 103 A2:Transport to Manufacturer 27 27 A3: Manufacturing 63 −21 Total (A1 +A2 + A3) 260 109

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to an object can include multiple objects unless thecontext clearly dictates otherwise.

As used herein, the terms “connect,” “connected,” and “connection” referto an operational coupling or linking. Connected objects can be directlycoupled to one another or can be indirectly coupled to one another, suchas via one or more other objects.

As used herein, the terms “substantially” and “about” are used todescribe and account for small variations. When used in conjunction withan event or circumstance, the terms can refer to instances in which theevent or circumstance occurs precisely as well as instances in which theevent or circumstance occurs to a close approximation. When used inconjunction with a numerical value, the terms can refer to a range ofvariation of less than or equal to ±10% of that numerical value, such asless than or equal to ±5%, less than or equal to ±4%, less than or equalto ±3%, less than or equal to ±2%, less than or equal to ±1%, less thanor equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a ratio in the rangeof about 1 to about 200 should be understood to include the explicitlyrecited limits of about 1 and about 200, but also to include individualratios such as about 2, about 3, and about 4, and sub-ranges such asabout 10 to about 50, about 20 to about 100, and so forth.

While the disclosure has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the disclosure asdefined by the appended claim(s). In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, operation or operations, to the objective, spirit and scope ofthe disclosure. All such modifications are intended to be within thescope of the claim(s) appended hereto. In particular, while certainmethods may have been described with reference to particular operationsperformed in a particular order, it will be understood that theseoperations may be combined, sub-divided, or re-ordered to form anequivalent method without departing from the teachings of thedisclosure. Accordingly, unless specifically indicated herein, the orderand grouping of the operations is not a limitation of the disclosure.

1. A manufacturing process of a low-carbon concrete product, comprising:forming a cementitious slurry including portlandite; shaping thecementitious slurry into a structural component, wherein shaping thecementitious slurry comprises compacting the cementitious slurry at apressure in a range of about 0.5 MPa to about 50 MPa; and exposing thestructural component to a post-combustion or post-calcination flue gasstream containing CO₂, thereby enabling manufacture of the low-carbonconcrete product.
 2. The manufacturing process of claim 1, whereinforming the cementitious slurry includes combining water and a binderincluding the portlandite and optionally cement and coal combustionresiduals at a water-to-binder mass ratio (w/b) of about 0.5 or less. 3.The manufacturing process of claim 2, wherein w/b is about 0.45 or less.4. The manufacturing process of claim 1, wherein forming thecementitious slurry includes combining water and a binder including acement, portlandite, and coal combustion residuals, at a mass percentageof the cement in the binder of about 25% or greater and up to about 50%.5. The manufacturing process of claim 4, wherein the mass percentage ofthe cement in the binder is about 30% or greater.
 6. The manufacturingprocess of claim 1, further comprising drying the structural componentprior to exposing the structural component to the post-combustion orpost-calcination flue gas stream containing CO₂.
 7. The manufacturingprocess of claim 6, wherein drying the structural component includesreducing a degree of pore water saturation (S_(w)) to less than
 1. 8.The manufacturing process of claim 7, wherein S_(w) is about 0.9 orless.
 9. The manufacturing process of claim 6, wherein drying thestructural component includes reducing S_(w) to a range of about 0.1 toabout 0.7.
 10. The manufacturing process of claim 6, wherein drying thestructural component is performed at a temperature in a range of about20° C. to about 85° C. for a time duration in a range of 1 h to about 72h.
 11. (canceled)
 12. The manufacturing process of claim 1, whereincompacting the cementitious slurry includes reducing Sw to less than 1.13. The manufacturing process of claim 12, wherein S_(w) is about 0.9 orless.
 14. The manufacturing process of claim 12, wherein compacting thecementitious slurry includes reducing S_(w) to a range of about 0.1 toabout 0.7.
 15. (canceled)
 16. The manufacturing process of claim 1,wherein exposing the structural component to the post-combustion orpost-calcination flue gas stream containing CO₂ is performed at ambientpressure and at a temperature in a range of about 20° C. to about 80° C.17. The manufacturing process of claim 1, wherein the low-carbonconcrete products have up to 75% lower embodied carbon intensity than atraditional cement-based concrete product.
 18. The manufacturing processof claim 17, wherein the lower carbon intensity is due to (a) partialsubstitution of cement with portlandite and/or fly ash and/or (b) CO₂uptake during manufacturing, 19-20. (canceled)